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Kerfing to introduce rocker into PVC foam core?

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faklord
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Kerfing to introduce rocker into PVC foam core?

Postby faklord » Tue Nov 01, 2016 5:47 pm

I'm considering using kerfing to set rocker into a 25mm PVC foam blank, for my next HF board.

The idea is:
1) Cut several deep grooves in the top surface, side to side, in the area where rocker is required.
2) Fill grooves with epoxy/micro-balloon mix
3) Set desired rocker, wipe away excess and leave to set.
4) Cut/shape desired shape from the preformed blank.

Some ball park values:
6 grooves, 1mm wide, 20mm deep, spacing around 100mm. The aim is about 10cm of rocker over the 1st 5-600mm of the board.

The main advantage of setting the rocker before lamination (rather that setting it during lamination, on a rocker table) is the top and bottom surfaces of the core don't change length as vac bagging pressure is applied. ie your nicely squeeged flat laminate is not disturbed.

Before jumping down this route I'd be interested to hear if this has been tried before and any real or perceived issues.

The two slight issues i can foresee are
a) The rocker curve may come out a little bit 'digital'. I can't see this being much an issue as it hopefully won't be in the water most of the time and its only really there to limit pearling on touchdowns.
b) There will be discontinuities in the core stiffness at the kerf lines. I can see this might be an issue if board was being designed to flex ...but the intention is to be stiff..top and bottom will include carbon. The kerf lines may also act as stress raisers but most the the (HF) stresses should be well behind them.

Comments & suggestions welcome....

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Re: Kerfing to introduce rocker into PVC foam core?

Postby plummet » Tue Nov 01, 2016 8:50 pm

Sounds interesting. Give it a go.

You can always shape it afterward to get rid of the "digital" look. But the digital look might look cool!...

I'm assuming you would use this method at the nose for a high rockered nose for touch and go?

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Re: Kerfing to introduce rocker into PVC foam core?

Postby revhed » Tue Nov 01, 2016 8:55 pm

I thought as you as did a 5 cm thick board of ultra cheap extruded poly styrene, grooves cut in 2 cm, 1 cm spacing, top and bottom, off set, full width of blank.
Did not fill openings.
Used top and bottom rocker table with lots of weight after laying up top and bottom.
So far so good.
R H

faklord
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Re: Kerfing to introduce rocker into PVC foam core?

Postby faklord » Tue Nov 01, 2016 9:31 pm

@ Plummet
"I'm assuming you would use this method at the nose for a high rockered nose for touch and go?"
Yup, that's the plan. Back end will be flat to minimize suck down drag.

@Revhed
Thanks for reply. Similar idea.
Once I've got the blank shaped I can do the layups in a Vac storage bag, without the need for a proper rocker table..just a couple of carefully positioned supports. This worked well on the last one (though on that one the core was two layers of 10mm laminated together to give the desired rocker)

I'll post up when its worked/failed

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Re: Kerfing to introduce rocker into PVC foam core?

Postby faklord » Tue Nov 15, 2016 11:02 pm

Well it looks like its worked ok so far:
IMG_2721.JPG
IMG_2722.JPG
IMG_2723.JPG
Rocker was simply set by clamping down the flat (back) part of the board and stuffing blocks of wood under the nose, while the epoxy mix set.
Initial rocker was set at 10cm but this sprung back to about 8cm when released. However, it was easy enough to repeat the process, just adding another couple of slots to reset it to a finished value of about 10cm. This (being able to rework the rocker) is quite a nice feature of this technique.
The 'digital rocker' is just about visible.
Hopefully this will be a tough core with "Almost no resin uptake" & "Almost no water absorption" (according to the material spec)
We'll see.....
So, onto vac bagging the laminates ....no rocker table required :D

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Re: Kerfing to introduce rocker into PVC foam core?

Postby rynhardt » Wed Nov 16, 2016 7:16 am

Very nice :thumb:

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Re: Kerfing to introduce rocker into PVC foam core?

Postby max » Wed Nov 16, 2016 11:04 am

faklord wrote: So, onto vac bagging the laminates ....no rocker table required :D

I would still put it on some sort of rocker setup . . . even if it is just weighted onto some curved support that matches the rocker.

The vac bag may create some extra rocker otherwise . . . maybe along the flat section . . . at least reduce the chance that this might happen.

Just my 2 cents worth . . . from one of my experiences.

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Re: Kerfing to introduce rocker into PVC foam core?

Postby faklord » Wed Nov 16, 2016 11:16 pm

max wrote:
faklord wrote: So, onto vac bagging the laminates ....no rocker table required :D

I would still put it on some sort of rocker setup . . . even if it is just weighted onto some curved support that matches the rocker.

The vac bag may create some extra rocker otherwise . . . maybe along the flat section . . . at least reduce the chance that this might happen.

Just my 2 cents worth . . . from one of my experiences.
Thanks for your comment...and I agree the vac bag could change the rocker or even introduce a twist!
I'll weight it onto a flat surface with a block under the nose...done this bit before...it is the method of pre-forming the rocker that is new (for me) this time.

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Re: Kerfing to introduce rocker into PVC foam core?

Postby skyte » Fri Nov 25, 2016 12:37 am

Really good idea!
You can do some cool things with kerfing...
http://www.grasshopper3d.com/video/kerf ... rasshopper

I've also seen something similar where the piece to be bent is split into 2 pieces... top and bottom. a sort of toothed / gear profile is cut into the mating faces of each piece such that when they mesh together they take on a curved form... Can't find the link for that at the moment but will post it if i find it! It gives a smoother result with less faceting but in your case a sander is all you need to get rid of the facetting!

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Re: Kerfing to introduce rocker into PVC foam core?

Postby plummet » Fri Nov 25, 2016 7:01 am

I like it. Yourl be able to do some high speed touch downs without nose diving.

What about chined railes for sideways touch downs?

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